Download Capacitor Leakage Measurements Using a Model 6517A Electrometer

Number 315
Application Note
Series
Capacitor Leakage Measurements
Using a Model 6517A Electrometer
Introduction
Capacitors are very important in all areas of electronics.
From timing circuits to sample and hold applications, we depend
on capacitors to act in a nearly ideal fashion. In many cases,
however, complex electrochemical interactions cause capacitors
to fall short of perfect.
One of the less ideal properties that a capacitor has is
leakage, or insulation resistance (IR). For a given dielectric
material, the effective parallel resistance is inversely proportional
to the capacitance. This is because the resistance is proportional
to the thickness of the dielectric, and inverse to the capacitive
area. The capacitance is proportional to the area and inverse to the
separation. Thus, a common unit for qualification of capacitor
leakage is the product of its capacitance and leakage resistance,
usually in megohm-microfarads (MΩ·µF).
possible to im-prove noise, yet small enough to avoid introducing
errors in the final result. This trade-off requires knowing the
approximate value of the insulation resistance, even before the
measurement has been made. Even then, finding the optimum
value for the series resistor can be difficult, especially when a
variety of capacitors are to be tested.
Second, the resistor adds noise to the measurement.
Johnson noise is thermal noise created by any resistor. At room
temperature this is roughly 6.5 × 10–10 ∆f/R amps, p-p. The
current noise in a 1TΩ feedback resistor at a typical 3Hz bandwidth would be ~8×10-16A. When measuring an insulation resistance of 1016Ω at 10V, the noise current will be 80% of the
measured current.
RF
RL
RS
Dielectric Types
For polymer dielectrics such as polystyrene, polycarbonate
or Teflon®, the insulation resistance can range from 104MΩ·µF to
108MΩ·µF, depending on the materials and their purity. For
example, a 1000pF Teflon cap with insulation resistance greater
than 1017Ω is specified as >108MΩ·µF. For various ceramics such
as X7R or NPO, insulation resistance can range from 103MΩ·µF
to 106MΩ·µF. Electrolytic capacitors such as tantalum or aluminum have much lower leakage resistances, in the region from
1MΩ·µF to 100MΩ·µF. For example, a 4.7µF aluminum cap
specified as 50MΩ·µF is guaranteed to have at least 10.6MΩ
insulation resistance.
V
C
+
Test
Circuit
Gain: 1 +
Feedback
Electrometer
RF
RL
+ RS
1 + R L jω C
Gain at 3Hz
without RS :
1 + R F j ωC
Gain at 3Hz
with RS :
1 +
RF
RS
(Typically 104 - 108)
(Typically 104 - 106)
Typical Test Method
Figure 1.
When measuring extremely low leakage capacitors, there
are a number of things to keep in mind. Normally, a feedback
electrometer would be used as shown in Figure 1. The series
resistor (RS) in the measurement loop is necessitated by noise
considerations. Without the resistor, an electrometer in this
configuration would have a very high noise gain at high frequencies. This noise amplification is unacceptable. The series resistor
limits the AC noise to a maximum level, although it does make
the measurement more complex. Since RS appears in the denominator of the simplified gain equation for 3Hz, a larger resistance
decreases the AC noise gain. To make a measurement to 0.1%,
the series resistor must be less than 0.1% of the insulation
resistance to be measured. Thus, the resistor should be as large as
Alternate Test Circuit
Even better results will be obtained with the circuit of
Figure 2. The added resistance of diode DS allows RS to be
reduced to approximately 100kΩ. At the beginning of the
capacitor charge cycle, the current through the diode is relatively
high and the equivalent resistance of the diode is low, allowing
the capacitor to charge quickly. As the capacitor be-comes
charged, the current will decrease steadily and the diode resistance will increase, limiting the noise amplification automatically.
Typically, the diode can be a small-signal diode, such as the
1N914 or the 1N3595. Note some series resistance is still
required to prevent overload in case the capacitor is shorted.
The diode should be enclosed in a light-tight metal enclosure to eliminate photo-electric as well as electrostatic interference.
RL2
RL1
CF
V
C
+
Test
Circuit
Gain: 1 +
jωC + 1/RL1
jωC + 1/R
F
Gain at 3Hz:
Feedback
Electrometer
1 +
L2
C
CF
(Typically 1 - 1000)
Figure 2.
Figure 3 show some typical response curves using different
series resistors.
sequences involving required environmental conditions, the
6517A has the ability to simultaneously monitor temperature and humidity. This provides a record of conditions, and
allows for easier determination of temperature coefficients.
Automatic time-stamping of readings provides a further
record of time-resolved measurements.
3. An optional switch card will allow repeated testing of up to
10 capacitors to facilitate small batch testing for the laboratory environment.
4. The V/I mode simplifies the task of making a single
measurement. In this mode a resistance reading of V/I is
displayed, using its output voltage and measured current
reading. When setting up a measurement system, it is
convenient to have readings in a form that requires no
interpretation. This is helpful when using the instrument
without a computer interface.
Test Circuits
For statistical purposes, a quantity of capacitors must be
tested to produce useful data. Obviously, it is impractical to
perform these tests manually, so some sort of automated test
system is required. Figure 4 illustrates such a system, which
employs a Model 6517A Electrometer/Source, Model 7158 Low
Current Scanner Cards, and Model 7169A Form C Switch Cards.
The cards must be installed in a switching mainframe, such as a
Model 7002. A computer controls the instruments to perform the
tests automatically.
7169A Form C Switch Card
7158 Low Current Card
R
C
R
C
R
C
Figure 3. Typical response curves
Implementation
The 6517A Electrometer offers several advantages when
measuring capacitor leakage.
1. The 6517A contains a low noise, variable 1kV voltage
source for making high resistance measurements, with
built-in current limiting. For a given capacitor, a larger
applied voltage within the voltage rating of the capacitor,
will give a larger leakage current. Measuring a larger
current with the same intrinsic noise floor produces a
greater signal-to-noise ratio, and a more accurate reading.
For capacitors with high voltage ratings, a 1000V source is
supplied. Because the voltage source is continuously
variable, voltage coefficients are easily obtained.
2. Temperature and humidity can have a significant effect on
high resistance measurements. It therefore becomes important to regulate and measure these quantities. For test
LO
Voltage Source Output
Model 6517A
Electrometer
HI
LO
Picoammeter Input
HI
Figure 4. Capacitor leakage test system
In this test system, a single instrument, the Model 6517A,
provides both the voltage sourcing and low current measurement
functions. This instrument is particularly useful for this application because it can display either resistance or leakage current and
will source up to 1000V DC. Due to the limitation of the Model
7169A card, the amount of voltage sourced should not exceed
8002-ILC-3 Interlock Cable
A) Connections
WARNING:NO INTERNAL OPERATOR SERVICABLE PARTS,SERVICE BY QUALIFIED PERSONN
LO
8002A HIGH RESISTANCE TEST FIXTURE
INPUT
HI
V SOURCE
PREAMP OUT COMMON
!
250V PEAK
250V PEAK
OUT
!
7056
INTERLOCK
7078-TRX-3 Triax Cable
CAUTION:FOR CONTINUED PROTECTION AGAINST FIRE HAZARD,REPLACE FUSE WITH SAME TYPE
Model 6517A
Note: Set fixture mode switch
to picoammeter operation.
8607 Banana Plug Cables
R2
Warning: Connect
of fixture
to safety earth ground using
safety ground wire (supplied
with 8002A test fixture).
1
B) Equivalent Circuit
Guard
Plate
pA
–
LO
–
+
GND
RX
3
Input
Amplifier
RF
HI
2
LCR
Input
Amplifier
+
To A/D
Converter
S
R1
S
HI
V-Source
Out
V Source
LO
Lid Interlock
Model 8002A
Test Fixture
7158
Interlock
To Interlock
Detection
Circuits
S
PREAMP OUTPUT
COMMON
Normally closed relays disabled
1Ω
2V ANALOG OUTPUT
S
Figure 5. Connections for resistance measurements using Model 8002A
test fixture
Figure 6. Capacitance and IR measurement system
500V. If the maximum test voltage is only 110V, the 7169A card
can be replaced with the Model 7111 Form C Switch Card.
currents that can become a significant portion of a high resistance
measurement if not controlled.
One set of switches is used to apply the test voltage to each
capacitor in turn; a second set of switches connects each capacitor
to the picoammeter after a suitable soak period. This system tests
up to ten capacitors but is easily expanded to any reasonable
number.
Connections to the 8002A from the 6517A are shown in
Figure 5. Note that the 8002A requires the use of a 8002-ILC3
Interlocking Cable. The interlock feature of the 6517A and
8002A prevents voltage from the 6517A from being applied when
the lid of the 8002A is open. However, once a capacitor has been
charged to a high voltage, the capacitor should be discharged
prior to any handling and removal from the fixture in order to
prevent any electrical shock hazards.
After the capacitors have been tested, the voltage source
should be set to zero and some time allowed to discharge the
capacitors before they are removed from the fixture. Note that in
Figure 4 the capacitors have a discharge path through the
normally closed contacts of the relays.
To prevent electric shock, test connections must be configured such that the user cannot come in contact with the conductors, connections, or the DUT. Safe installation requires proper
shielding, barriers, and grounding to prevent contact with
conductors.
For single capacitor testing, the Model 8002A High
Resistance Test Fixture can be used with the 6517A. The Model
8002A has been specifically designed to minimize leakage
More complex test systems are possible, combining the
leakage measurement with capacitance measurements, dielectric
absorption and other tests, if desired. A simplified schematic of
such a test system using an LCZ bridge and a picoammeter with a
voltage source is shown in Figure 6.
Example Program and Description
An example computer program is shown in Figure 7. This
example program illustrates the programming of the 6517A using
the built-in test sequence, Capacitor Leakage Test, of the 6517A
that provides a capacitor leakage measurement.
Refer to the program listing for the following program
description.
After the 6517A has been cleared, the instrument is
configured for the following functions:
• Math calculations turned off.
• 6517A set to SRQ when buffer is full.
• Retum ASCII data for the leakage current reading and the
Voltage Source value.
• Set the 6517A to measure current.
• Set the 6517A for autoranging.
Once these commands are issued, the program checks to
see if the operation has been completed prior to sending any
further commands. This gives the 65l7A the appropriate amount
of time to be configured.
After the initial setup, the operator is then prompted for the
desired soak voltage, the soak time, and the discharge time. The
discharge time allows the capacitor to be discharged so that it
may be handled. With these values entered, the program then
sends the test sequence commands to the 6517A with the
prompted values for the test. Again, once these commands are
issued, the program checks to see if the operation has been
completed prior to sending any further commands. This gives the
6517A the appropriate amount of time to be configured.
When the operation complete function has been satisfied,
the program then arms the test sequence to begin the test. During
the testing phase, the computer program waits for the SRQ to be
identified when the data buffer has been filled. When the SRQ
has been identified, the program reads the measurement status
register. This action clears the SRQ from the status register.
Clearing the SRQ is important, especially if the test is run again.
In addition to clearing the status register, getting the data from the
buffer clears the event that caused the SRQ.
The data is obtained and placed into the variable CapLeak$.
Because the data is returned in ASCII format and there are two
values associated with the single capacitor leakage current value,
the string must be parsed in order to separate the current reading
and the voltage source values. As shown in the example, BASIC
uses the MIDS$ function to obtain a subset ofthe string.
Although this example was written with using BASIC in
mind, the program can be modified for Visual Basic, C,
TestPoint, LabVIEW, or any other programming environment
that supports GPIB communication with instruments. Examples
in TestPoint and LabVIEW are available from Keithley at
www.keithley.com.
Test System Safety
Many electrical test systems or instruments are capable of
measuring or sourcing hazardous voltage and power levels. It is
also possible, under single fault conditions (e.g., a programming
error or an instrument failure), to output hazardous levels even
when the system indicates no hazard is present.
These high voltage and power levels make it essential to
protect operators from any of these hazards at all times. Protection methods include:
• Design test fixtures to prevent operator contact with any
hazardous circuit.
• Make sure the device under test is fully enclosed to
protect the operator from any flying debris.
• Double insulate all electrical connections that an operator
could touch. Double insulation ensures the operator is
still protected, even if one insulation layer fails.
• Use high-reliability, fail-safe interlock switches to
disconnect power sources when a test fixture cover is
opened.
• Where possible, use automated handlers so operators do
not require access to the inside of the test fixture or have
a need to open guards.
• Provide proper training to all users of the system so they
understand all potential hazards and know how to protect
themselves from injury.
It is the responsibility of the test system designers, integrators, and installers to make sure operator and maintenance
personnel protection is in place and effective.
Figure 7: Software Example Listing
Sub SetupInst()
'Send Reset command to 6517A. 6517A at Address 27.
Call Send(27, "*RST", status%)
'Set 6517A initial measurement functions, range and measurement elements.
Call
Call
Call
Call
Send(27,
Send(27,
Send(27,
Send(27,
"*CLS", status%)
":Calc1:Stat Off",status%)
":Stat:Meas:Enab 512;*SRE 1", status%)
":Form:Data ASC;Elem Read,Vso", status%)
Call
Call
Call
Call
Send(27, ":Sens:Func 'Curr", status%)
Send(27, ":Sens:Curr:Rang:Auto On", status%)
Send(27, "*Opc?”, status%)
Enter(OpCom$, 20, 1%, 27, status%)
'Clear all status registers
'Disable Math calculations
'Set to SRQ on Buffer Full,
'Return readings as ASCII values
'floating point format and only return the
'readings and the Vsource value.
'Set measurement mode to Current
'Set for autoranging
'Query for Operation Complete
'Get Operation Complete Bit
End Sub
Sub TakeReading()
Input "Input Soak Voltage" SoakVoltage
Input "Input Soak Time"; SoakTime
Input "Input Discharge Time"; DischargeTime
'Enter Soak Voltage
'Enter Soak Time
'Enter Discharge Time
'Enter values into Test Sequence Commands and run test
Call Send(27, ":Trace:Clear", status%)
'Clear contents of trace buffer
Call Send(27, ":Tseq:Type CLE", status%)
'Set test sequence for Cap Leakage
Call Send(27, ":Tseq:CLE:Svol " +str$(SoakVoltage), status %)
'Set Soak Voltage for sequence
Call Send(27, ":Tseq:CLE:Stime "+str$(SoakTime), status%) 'Set Soak Time for sequence
Call Send(27, ":Tseq:CLE:Dtime "+str$(DischargeTime), status%)
'Set Discharge Time for sequence
Call Send(27, "Tseq:Tso Imm", status%)
Call Send(27, "*Opc?", status%)
Call Enter(OpCom$, 20, 1%, 27, status%)
'Enable Immediate start of sequence
'Query for Operation Complete
'Get Operation Complete Bit
'Arm and start sequence
Call Send(27, ":Tseq:Arm", status%)
'Start Seqeunce
'At this point, the program waits for the SRQ from the 6517A when the data buffer is full.
WaitSRQ:
IF (NOT (srq%)) THEN GOTO WaitSRQ
'When the SRQ is identified, execute the following code:
Call Send(27, ":Stat:Meas:Event?", status%)
Call Enter(poll$, 50, 1%, 27, status%)
'Get status and clear status
'Get poll value
Call Send(27, ":Trace:Data?", status%)
Call Enter(CapLeak$, 30, 1%, 27, status%)
'Query for buffered readings
'Get Reading and Vsource
'Since there are two values returned, the reading and the Vsource value, the CapLeak$
'must be parsed out to get the two values. The data sent back to the host computer will
'be in this format:
'
'
(+/-)1234.567E(+/-)00,(+/-)0010.000(LF)
'
'Note that (+/-) represents one character. Thus the 6517A returns a 'total of 24 characters
'including the (,) comma and the (LF) Line Feed character.
PRINT "Leakage Current Value=";MID$(CapLeak$, 1, 13)
PRINT "Soak Voltage=";MID$(CapLeak$, 15, 13)
End Sub
'Extract 13 characters for the leakage value
'Extract next 13 characters after comma for
'VSource value.
Specifications are subject to change without notice.
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